The invention relates to a pulse radar apparatus comprising a transmitter and receiver, whereby the target returns--after detection--are sampled and digitised per range quant. An n-point FFT processing unit and a threshold circuit are provided for determining a threshold value for each of the n frequency output channels of the FFT processing unit, above which threshold value the output signals of the FFT processing unit are passed.
The FFT processing unit converts the video data, sampled and digitised per range quant, from n successive radar scans into n output signals situated in adjoining frequency bands. Such a conversion is hereinafter referred to as an FFT scan. A division of the radar range into radar cells formed by range quants and azimuth sectors determined by n successive azimuth scans then corresponds with a division into FFT cells formed by FFT scans and range quants. For the successive FFT scans the corresponding azimuth sectors may overlap each other partly; hence, in such a case the radar cells of these azimuth sectors will also overlap each other. The output signals of the FFT processing unit determine the spectrum of the target returns processed per radar cell. This spectrum will of course differ for fixed and moving targets, where the fixed targets include targets having a small radial velocity component. With the radar beam moving over the target, the principal frequency components from the spectrum of the returns of fixed targets, for successive radar cells in a range bin, differ mutually in the sense that with the use of a single canceller in the receiver of the pulse radar apparatus the frequency components will be of minimum value at the instant the beam is directed at the target centre and will increase in value as the beam is directed away from the centre. This effect is less pronounced with the use of a double canceller. The spectra of the returns from moving targets, for successive radar cells in a range bin, may differ in the sense that the frequency components will be of maximum value at the instant the beam is directed at the target centre and will decrease in value as the beam is directed away from the centre. Because of these effects, the decision regarding whether target returns are from clutter or from targets having a velocity that corresponds with a frequency in the order of k.prf (k=0, 1, 2, . . . ) may differ, depending how closely the radar beam is directed at the target center. If the range quants are selected smaller than the length of the target returns, several successive digitised samples will be obtained from such returns. Because of the random noise present, the distinction between clutter and moving targets in the cells succeeding each other in range may in general come out differently as well.